1. Field of the Invention
The present invention generally relates to an optical-fiber preform and, more particularly, to a device for providing a raw material during a chemical-vapor deposition process.
2. Description of the Related Art
Optical communication, which uses an optical fiber as a transmission medium, can transfer a vast amount of information at a faster rate than an electric communication system. Also, better quality communication is possible as the optical communication is free of interference from radios or magnetic fields.
For fabricating an optical fiber, which is drawn from an optical preform, there are sol-gel method and vapor-phase deposition method. The sol-gel method involves converting a liquid raw material into a mold to gelify the liquid raw material, then sintering the gel material to manufacture a silica glass. As the entire process occurs at low temperature, the production cost is low and relatively easy to make an adjustment in the composition of the fiber.
There are different types of vapor-phase deposition method, including a modified-chemical-vapor deposition (MCVD), vapor-phase-axial deposition (VAD), or outside-vapor deposition (OVD). These vapor-phase deposition methods involve heating a designated target rod or a glass tube and, at the same time, supplying raw material in a gas state to produce a suit on the target rod or on the tube, thus producing an optical fiber preform. As a part of the vapor-phase deposition process, the liquid raw material is vaporized in a designated device for holding the raw material and later supplied to an optical-fiber preform deposition path.
FIG. 1 is a simplified diagram of the conventional device 100 for holding the raw material during the chemical-vapor deposition process. Particularly, FIG. 1 introduces a device including a level sensor for sensing the liquid level of the raw material inside a bubbler, thereby sensing whether the raw material has been provided or cut off. As depicted in the drawing, the conventional raw material-providing device 100 for a chemical-vapor deposition process provides carrier gas B to the raw material inside the bubbler 110 to vaporize the liquid raw material to a gas state C, then the vapor is provided to a deposition unit. As the liquid raw material inside the bubbler 110 is provided to the deposition unit after going through the phase change, the bubbler 110 is replenished with raw material. The bubbler 110 includes a raw material providing a pipe 120 for receiving raw material A, a gas pipe 130 for providing carrier gas B, and a vapor pipe 140 for providing the phase change raw material C to the deposition unit. To maintain the temperature of the raw material inside the bubbler 110 uniformly, a thermal sensor 113 and a heater 115 are provided in the bubbler 110.
A level sensor 111, which is mounted in the bubbler 110, is provided for use in maintaining the amount of raw material inside the bubbler 110 within a certain level. As the raw material inside the bubbler 110 is gradually decreased, the level sensor 111 generates a signal for controlling whether the flow of the raw material should be continued or cut off.
However, there is a drawback in the conventional method when the level sensor is used for sensing the amount of raw material inside the bubbler before providing the raw material. In particular, when the raw material is being provided or when the raw material goes through the phase change, the disturbance caused by the raw material inside the bubbler tends to cause the level sensor to malfunction. In addition, the conventional device lowers the overall work efficiency as the deposition process cannot be conducted at the time when the raw material is being received.
Another drawback of the conventional device using a level sensor is that it is difficult to maintain a constant amount of the raw material inside the bubbler because the raw material is designed to replenish whenever the amount of the raw material inside the bubbler is under a certain level. As the amount of the phase-change raw material (i.e., liquid to vapor) is not uniform, it is hard to maintain a constant vapor pressure, and the vapor amount that needs to be provided to the deposition unit is not uniform. As a result, these factors tend to deteriorate the quality of the optical-fiber preform being deposited.
FIG. 2 is a simplified diagram of the conventional device 200 used in a chemical-vapor deposition process, which incorporates a mechanical valve that opens/closes, depending on the weight of the raw material inside a bubbler. As depicted in FIG. 2, the prior-art device 200 includes an external bubbler 210a and an internal bubbler 210b. The internal bubbler 210b is provided with carrier gas B′ and vaporizes the liquid raw material to a gas state C′ before providing it to a deposition unit. Meanwhile, as the vaporized raw material inside the external and the internal bubblers 210a, and 210b is provided to the deposition unit, the bubblers 210a and 210b are replenished with raw material. To this end, the bubbler 210a, 210b includes a raw material providing pipe 220 for receiving raw material A′, a gas pipe 230 for providing carrier gas B′, and a vapor pipe 240 for providing the phase change raw material C′ to the deposition unit. In addition, to maintain the temperature of the raw material inside the bubbler 210a and 210b uniformly, a thermal sensor 213 and a heater 215 are further provided in the bubbler 210a and 210b. As such, the internal bubbler 210b gradually rises to the surface due to the internal pressure of the providing pipe 220 when the raw material is consumed and eventually opens a pressure-operated valve 225 to provide the raw material to the bubbler 210a and 210b. More details on such raw material-providing device are disclosed in the U.S. Pat. No. 5,938,985.
However, as the pipe used for providing the raw material using a pressure-operated valve must maintain constant pressure therein, the pipe can be easily damaged. In addition, undesirable pollutants can be created between the pressure-operated valve and the inflow opening of the pipe due to mechanical friction.